1. Field of the Invention
The present invention relates to a mobile communication terminal, and more particularly, to a method and an apparatus for managing neighbor cells in a Global System for Mobile Communications (GSM).
2. Description of the Related Art
2nd Generation (2G) mobile telecommunication systems that provide voice-based services include for example a Global Systems for Mobile Communications (GSM), an Interim Standard (IS)-95. The GSM was commercialized in Europe in 1992 and is based on a Time Division Multiple Access (TDMA).
Referring to FIG. 1, communication between a base station and a mobile terminal in the GSM system includes transmission of a TDMA frame, referred to as a physical channel. The TDMA frame 101 consists of 8 time slots. Data transmitted during one time slot is referred to as a burst. Different logical channels are mapped to the physical channel. The logical channel includes a traffic channel and a control channel. A traffic channel (TCH) multi-frame (MF) 103 consists of 26 TDMA frames 101, and a control channel multi-frame 105 consists of 51 TDMA frames 105. Each of super frames 107 and 109 may consist of 1326(=26×51) TDMA frames 101. The super frames 107 and 109 can be combined to form a hyper frame 111 consisting of 2048 super frames.
The traffic channel is used to transmit voice and data at a data transfer rate of up to 13 Kbps, and the control channel manages a GSM system signaling. Specifically, the control channel is used to carry signaling and synchronization data. The control channels are classified into a broadcast channel (BCH), a common control channel (CCCH), and a dedicated control channel (DCCH).
The BCH is subdivided as follows:
(1) Frequency correction channel (FCCH): This channel is transmitted from the base station to the mobile terminal and used to provide information for frequency correction of the mobile terminal.
(2) Synchronization channel (SCH): This channel is transmitted from the base station to the mobile terminal and used to provide frame synchronization information of the mobile terminal (number of TDMA frames) and base station identification code (BSIC) information for identifying the base station.
(3) Broadcast control channel (BCCH): This channel is transmitted from the base station to the mobile terminal and used to provide information about a selected cell, a neighbor cell and related information for cell selection, and system information.
The types of bursts include a frequency correction burst (FCB), a synchronization burst (SB), and a normal burst (NB). Structures of the FCB, the SB, and the NB will be described below with reference to FIGS. 2A to 2C.
Referring to FIG. 2A, the FCB consists of two 3-bit tail periods, a 142-bit synchronization sequence period each bit of which is zero, and an 8.25-bit guard period. The FCB is used for frequency synchronization of the mobile terminal. The repetition of the FCB defines the FCCH.
Referring to FIG. 2B, the SB consists of two 3-bit tail periods, two 39-bit coded data periods in which the information about the number of frames is encoded, a 64-bit synchronization period, and an 8.25-bit guard period. The FCB is used for time synchronization of the mobile terminal. The repetition of the SB defines the SCH.
Referring to FIG. 2C, the NB consists of two 3-bit tail periods, two 1-bit stealing flags (S), two 57-bit coded data periods, a 26-bit training sequence period, and an 8.25-bit guard period. The NB is used to carry data on the TCH and all control channels other than a random access channel (RACH), the FCCH, and the SCH.
A conventional method for managing a GSM neighbor cell of a mobile communication terminal will be described below. First, a GSM terminal has to maintain synchronization with respect to six neighbor cells transmitted on a BCCH allocation (BA) list. In other words, the mobile terminal is roughly synchronized with the neighbor cells through the FCB, and obtains system information and more accurate synchronization information from the neighbor cells through the SB. Thus, the mobile terminal can exchange actual data with the neighbor cells through the NB. The mobile terminal has to acquire and maintain the synchronization with the corresponding neighbor cells by sequentially detecting and decoding the FCB and the SB transmitted from the neighbor cells.
The mobile terminal decodes the entire BCH data of a serving cell at least every 30 seconds, and decodes the BCCH data block of BCCH carriers of the six neighbor cells at least every 5 minutes. When a new BCCH carrier becomes one of the six neighbor cells, the mobile terminal has to decode the BCCH data within 30 seconds. Upon failure, the mobile terminal discards the BCCH data.
In addition, the mobile terminal synchronizes the FCB and the SB with BCCHs of the six neighbor cells at least every 30 seconds. In this case, even though the mobile terminal fails to acquire the synchronization with the neighbor cells, the mobile terminal acquires the synchronization with the corresponding cells by decoding BCH (FCB/SB) information at least every 30 seconds. However, when the signal electric field such as a received signal strength or Rx quality level is poor, there is a very high probability that the FCB-SB-BCCH data decoding for acquiring synchronization with the neighbor cells will fail. Upon failure of the synchronization acquisition, the mobile terminal continues to try to acquire the synchronization with the neighbor cells until the neighbor cells are discarded from the neighbor cell list (the BA list). In this case, the mobile terminal continues in its attempts to acquire the synchronization with the neighbor cells during an idle mode, causing an increase in power consumption of the mobile terminal.